In general, copper electric wires are used in wiring harnesses which are laid out on a vehicle such as an automotive vehicle. In connecting wiring harnesses or a wiring harness with onboard equipment, terminals are attached to the copper electric wires of the wiring harness, and terminals of this type are generally attached to the copper electric wires through press-clamping.
Typically, the terminal which is press-clamped to the copper electric wire includes a bottom plate portion on which a conductor portion of the copper electric wire which is made up of a plurality of twisted strands of copper is placed and a pair of conductor caulking pieces which are provided consecutively to the bottom plate portion for holding therebetween the conductor portion placed on the bottom plate portion. Then, the conductor caulking pieces are caulked to the conductor portion in such a manner that distal end portions of the conductor caulking pieces are inserted into the conductor portion, so as to produce a state in which the strands of the conductor portion are partially embraced by the conductor caulking pieces, whereby the terminal is securely press-clamped to the conductor portion of the copper electric wire.
In recent years, in consideration of weight reduction of vehicles and easiness of recycling as well as shortage of copper resources, aluminum electric wires have been collecting many people's attention. However, in an aluminum electric wire when compared with copper, an oxide layer formed on an aluminum surface becomes thick, and there is a tendency of contact resistance between a conductor portion and a terminal becoming relatively high. As a means for reducing the contact resistance, there has been known a method in which respective conductor caulking pieces of a terminal are caulked strongly to the conductor portion so as to increase the compressibility factor of the conductor portion. According to this method, oxide layers on the respective strands making up the conductor portion are broken, whereby the contact resistance between the conductor portion and the terminal is reduced. Note that when used in this specification, the compressibility factor of the conductor portion is regulated as a ratio of a sectional area of the conductor portion before press-clamping to a sectional area of the conductor portion after press-clamping.
However, stress applied to the conductor portion increases as the compressibility factor of the conductor portion increases. In addition, aluminum is inferior in mechanical strength to copper, and in the aluminum electric wire, when too large stress is laid on the conductor portion, the press-clamping strength of the terminal is deteriorated remarkably. Then, in press-clamping between an aluminum electric wire and a terminal, there have been proposed various press-clamping structure for press-clamping between an aluminum electric wire and a terminal for the purpose of making the reduction in contact resistance between the aluminum electric wire and the terminal with ensuring the press-clamping strength of the terminal (for example, refer to Patent Documents 1 and 2).
In a press-clamping structure disclosed in Patent Document 1, a compressibility factor which can obtain compatibility between reduction in contact resistance between a conductor portion of an aluminum electric wire and a terminal and ensuring press-clamping strength of the terminal is regulated in accordance with a sectional area of the conductor portion. For example, a compressibility factor of a conductor portion whose sectional area is less than 1.5 mm2 is referred to as 50 to 70%, while a compressibility factor of a conductor portion whose sectional area is equal to or more than 1.5 mm2 is referred to as 40 to 70%. In addition, a compressibility factor of a conductor portion in press-clamping of a copper electric wire to a terminal is typically referred to as 75 to 95%.
In a press-clamping structure disclosed in Patent Document 2, as is shown in FIGS. 5A and 5B, there are provided two pairs of caulking pieces (barrels) that are to be caulked on to a conductor portion 202 of an aluminum electric wire 201. In addition, holding caulking pieces 203 and electricity conducting caulking pieces 204 are caulked on to the conductor portion 202 by the use of a caulking tool in which a difference in level is provided in such a manner that the holding caulking pieces 203 which are situated on a proximal end side of the conductor portion 202 become higher than the electricity conducting caulking pieces 204 which are situated on a distal end side of the conductor portion 202 in such a state that both the caulking pieces are so caulked. By this configuration, a compressibility of the conductor portion 202 by the holding caulking pieces 203 is made to be substantially the same as a compressibility of a conductor portion of a copper electric wire in press-clamping of a terminal to the copper electric wire so as to ensure press-clamping strength while a compressibility factor of the conductor portion 202 by the electricity conducting caulking pieces 204 is made to be relatively high so as to reduce contact resistance.
[Patent Document 1] JP-A-2005-174896
[Patent Document 2] JP-A-2005-50736
In the press-clamping structure disclosed in Patent Document 1, the compressibility factor of the conductor portion is increased overall, compared with the case of copper wire, and hence, it is inevitable that the press-clamping strength is reduced. In contrast to this, in the press-clamping structure disclosed in Patent Document 2, the compressibility factor of the conductor portion can be made to differ between the location where the holding caulking pieces are caulked and the location where the electricity conducting caulking pieces are caulked, whereby the compressibility factor of the conductor portion in the location where the holding caulking pieces are caulked is made to be substantially the same as that of copper wire, so as to avoid the reduction in press-clamping strength of the terminal. However, in the press-clamping structure disclosed in Patent Document 2, the special caulking tool is necessary to caulk both the caulking pieces to the conductor portion in such a manner that the electricity conducting caulking pieces differs in height from the holding caulking pieces, and additionally, the heights at the two locations of the electricity conducting caulking pieces and the holding caulking pieces need to be managed, and to do this, costs have to be incurred exclusively.
In addition, it is generally understood that spring back is produced in the caulking pieces which are caulked on to the conductor portion. In the press-clamping structures disclosed in Patent Documents 1 and 2, the distal end portions of the caulking pieces are inserted into the conductor portion in such a manner that their projecting end faces are oriented towards the bottom plate portion. In the case of the caulking pieces which are caulked in the way described therein, the distal end portions are easily dislocated from the conductor portion, and spring back is easily produced. In addition, in the case of the conductor portion of the aluminum electric wire, aluminum which constitutes the respective strands is easier to produce stress relaxation than copper, and even in the event that spring back is produced in the caulking pieces to reduce the stress, it is difficult for them to restore the original positions. Due to this, a gap is produced between the caulking pieces and the conductor portion, whereby there is caused a risk that desired contact resistance and press-clamping strength cannot be obtained.